324 _fOCUS
Cytokine expression ~e~o~~~~a~t viruses - a new vaccine strategy tan A. Ramshaw, Janet Ruby and Alistair Rarnsay The discovery that the immune system is activated and regulated by hormone-like cytokincs has opened up new opportunities for designing recombinant vaccines. Live vaccine vectors which encode cytokines may increase the safety ofviral vectors and permit the manipulation of the immune response so that ef?ective protective immunity can be induced.
cytokines which are secreted from infected cells, and thus are able to intlucncc the immune response. In this article we discuss the potential ofthis approach for the development of safe and effective live vaccines for boosting the immune system, and show how this strategy has provided unique insights into the role of cytokines in virus infections.
Cytokines and the immune response Cytokines arc signalling molecules involved in communication between cells, mainly those of the immune system, that determine not only the magnitude, but also the type of immune response produced (see Table 1). It now appears that the immunity induced by an infectious agent is determined, in part, by the pattern of cytokines secreted by different subsets of T Iymphocytes’J. One subset (THI) secretes the cytokines interleukin 2 (IL-Z) and inrerteron-y (IFN-y) and promotes cell-mediated immunity, which is considered to be important in immune defense against intracellular parasites (e.g. viruses), whilst another population (TH2) secretes IL-4, IL-S, IL-6 and IL-10 and preferentially induces antibody responses (humoral immunity), with selective production of IgGl, I$ and IgA isotypes3-“. Cytokincs also influence the development of these different T-cell subpopulations. For example, IL-4 and IL-lo, produced by the TH2 subset, inhibit ehe development of THi populations, whilst THI -derived IFN-y suppresses the TH2 response. This may account for the invctic relationship often seen between humoral and cellmediated immunityh~‘. The extensive range of potential immune responses to an infectious agent is therefore tightly regulated by the cytokines produced by T lymphocytes. Given this information, it is not surprising that attempts have bccc c:adc to modify the immune response by the administration of recombinant cytokincs. However. these factors gcncral!y have a very short half-life in virtn, and there arc major dificultics associated with toxicity and in targeting cptokines to sites of immune reactivity. We have attempted to overcome these problems by constructing recombinant viruses that encode cytokines. During replication in vim, these viruses product the
Improving the safety of live viral vectors Live recombinant viral vaccines offer several significant advantages over inactivated or subunit vaccine?+,‘. Important among these is the ability of recombinant viruses to induce a strong cell-mediated immunity which may be essential to prevent infection with some pathogens. A number ofviruses including adenovirus and herpes virus have been described as potential vectors for expressing foreign antigens. However, by far the most widely studied and the strongest candidate is vaccinia virus. Vaccinia was used in the global eradication of smallpox and there is therefore extensive epidemiological information on its use as a vector. The success of the smallpox campaign was due, in part, to some of the properties of the virus: it is extremely stable when freeze-dried, the cost ofproduction is low and it can be administered by simple dermal abrasion. Drawbacks in using vaccinia virus as a vector include the variability of the immune response with different foreign antigens, and the serious side-et%cts that can occur tf the virus is inadvertently given to immunasuppre;scd individuals”‘. The latter presents a major problem in view cf the increased incidence of immunodeficiency caused by the human immunodeficiency viruses (HIV). Current attempts to diminish virus virulence include deleting certain non-zssential genes, e.g. thymidine kinasc (TK); howcvcr, the reduction in virulence has been accompanied by an undesirable loss of immunogenicity. Other poxviruscs with restricted animal-host ranges are also bring tcstcd as vaccine vectors and appear to !-e sat?, as determined in laboratory animals”. 13~ constructing recombinant viruses which encode cytokincs, we have shown that it is possible not only to attenuate the virus vector but also selectively to stimulate particular immune responses important in h?st resistance. Expression ofcytokines by recombinant
vaccinia
viruses
WC have constructed which cncodc’different
rccomhinant vaccinia viruses cytokine genes as wrll as the
_i_..__
TIBTECH DtICEMBER 1992 WOL lo,
Q 1992.Elsetier Science Publishers
Ltd IUKJ
42s J
focus
haemagglutinin (HA) gene of influenza virus. HA is encoded in these viruses as a reporter antigen to allow us to monitor the induction ofspecific immunity to a co-expressed protein’?. The cytokines that have been studied to date fall into two categories based on their e&ct on the growth and immunogcnicity of the vector: (1) those that alter the pathogencsis of the virus; and (2) those that selectively stimulate specific immune responses. Both of these properties are relevant to the development ofsafc and effective vaccines. The effects of cytokinc expression by these recombinant viruses on the immune response are summarized in Table 1.
Before vaccinia virus can bc used widely as a vaccine vector it will be necessary to rni;dify its biological properties to prevent the rare, but sometimes serious, side-effects observed when the virus was used as a vaccine against smallpox. These included genel-al&d dissemination of the virus and/or infection of :he central nervous system. Similar effects are found when the virus is administered to immunosup~~ressed mice. Athymiz nude mice are genetically deficient in T lymphocytes and are therefore unable to mount an efEcrive humorai or cell-mediated immune response. infection of rmde mice with vaccinia virus is normally lethal, the mice dying within ?&-?2 days from disseminated viral disease. However, vaccinia viruses encoding IL-2, IFN-y or tumour necrosis factor (TNF) are highly attenuated, such that nude mice arc readiiy able to control the growth ofthe virus, i.e. the virus is self-limiting (Fig. I)‘“‘“. Interestingly, although nude mice are immunodeficient, they are able to resist a challenge with influenza virus when previously vaccinated with a recombinant vaccinia virus encoding both IL-2 and the HA gene of influenza virusis. Although the growth of these cytokine-encoding viruses is also highly attenuated in normal mice, i.e. the viruses grow to lower titres and cause none of the pathology associated with v,
Cytokine expression ~e~o~~~~a~t viruses - a new vaccine strategy tan A. Ramshaw, Janet Ruby and Alistair Rarnsay The discovery that the immune system is activated and regulated by hormone-like cytokincs has opened up new opportunities for designing recombinant vaccines. Live vaccine vectors which encode cytokines may increase the safety ofviral vectors and permit the manipulation of the immune response so that ef?ective protective immunity can be induced.
cytokines which are secreted from infected cells, and thus are able to intlucncc the immune response. In this article we discuss the potential ofthis approach for the development of safe and effective live vaccines for boosting the immune system, and show how this strategy has provided unique insights into the role of cytokines in virus infections.
Cytokines and the immune response Cytokines arc signalling molecules involved in communication between cells, mainly those of the immune system, that determine not only the magnitude, but also the type of immune response produced (see Table 1). It now appears that the immunity induced by an infectious agent is determined, in part, by the pattern of cytokines secreted by different subsets of T Iymphocytes’J. One subset (THI) secretes the cytokines interleukin 2 (IL-Z) and inrerteron-y (IFN-y) and promotes cell-mediated immunity, which is considered to be important in immune defense against intracellular parasites (e.g. viruses), whilst another population (TH2) secretes IL-4, IL-S, IL-6 and IL-10 and preferentially induces antibody responses (humoral immunity), with selective production of IgGl, I$ and IgA isotypes3-“. Cytokincs also influence the development of these different T-cell subpopulations. For example, IL-4 and IL-lo, produced by the TH2 subset, inhibit ehe development of THi populations, whilst THI -derived IFN-y suppresses the TH2 response. This may account for the invctic relationship often seen between humoral and cellmediated immunityh~‘. The extensive range of potential immune responses to an infectious agent is therefore tightly regulated by the cytokines produced by T lymphocytes. Given this information, it is not surprising that attempts have bccc c:adc to modify the immune response by the administration of recombinant cytokincs. However. these factors gcncral!y have a very short half-life in virtn, and there arc major dificultics associated with toxicity and in targeting cptokines to sites of immune reactivity. We have attempted to overcome these problems by constructing recombinant viruses that encode cytokines. During replication in vim, these viruses product the
Improving the safety of live viral vectors Live recombinant viral vaccines offer several significant advantages over inactivated or subunit vaccine?+,‘. Important among these is the ability of recombinant viruses to induce a strong cell-mediated immunity which may be essential to prevent infection with some pathogens. A number ofviruses including adenovirus and herpes virus have been described as potential vectors for expressing foreign antigens. However, by far the most widely studied and the strongest candidate is vaccinia virus. Vaccinia was used in the global eradication of smallpox and there is therefore extensive epidemiological information on its use as a vector. The success of the smallpox campaign was due, in part, to some of the properties of the virus: it is extremely stable when freeze-dried, the cost ofproduction is low and it can be administered by simple dermal abrasion. Drawbacks in using vaccinia virus as a vector include the variability of the immune response with different foreign antigens, and the serious side-et%cts that can occur tf the virus is inadvertently given to immunasuppre;scd individuals”‘. The latter presents a major problem in view cf the increased incidence of immunodeficiency caused by the human immunodeficiency viruses (HIV). Current attempts to diminish virus virulence include deleting certain non-zssential genes, e.g. thymidine kinasc (TK); howcvcr, the reduction in virulence has been accompanied by an undesirable loss of immunogenicity. Other poxviruscs with restricted animal-host ranges are also bring tcstcd as vaccine vectors and appear to !-e sat?, as determined in laboratory animals”. 13~ constructing recombinant viruses which encode cytokincs, we have shown that it is possible not only to attenuate the virus vector but also selectively to stimulate particular immune responses important in h?st resistance. Expression ofcytokines by recombinant
vaccinia
viruses
WC have constructed which cncodc’different
rccomhinant vaccinia viruses cytokine genes as wrll as the
_i_..__
TIBTECH DtICEMBER 1992 WOL lo,
Q 1992.Elsetier Science Publishers
Ltd IUKJ
42s J
focus
haemagglutinin (HA) gene of influenza virus. HA is encoded in these viruses as a reporter antigen to allow us to monitor the induction ofspecific immunity to a co-expressed protein’?. The cytokines that have been studied to date fall into two categories based on their e&ct on the growth and immunogcnicity of the vector: (1) those that alter the pathogencsis of the virus; and (2) those that selectively stimulate specific immune responses. Both of these properties are relevant to the development ofsafc and effective vaccines. The effects of cytokinc expression by these recombinant viruses on the immune response are summarized in Table 1.
Before vaccinia virus can bc used widely as a vaccine vector it will be necessary to rni;dify its biological properties to prevent the rare, but sometimes serious, side-effects observed when the virus was used as a vaccine against smallpox. These included genel-al&d dissemination of the virus and/or infection of :he central nervous system. Similar effects are found when the virus is administered to immunosup~~ressed mice. Athymiz nude mice are genetically deficient in T lymphocytes and are therefore unable to mount an efEcrive humorai or cell-mediated immune response. infection of rmde mice with vaccinia virus is normally lethal, the mice dying within ?&-?2 days from disseminated viral disease. However, vaccinia viruses encoding IL-2, IFN-y or tumour necrosis factor (TNF) are highly attenuated, such that nude mice arc readiiy able to control the growth ofthe virus, i.e. the virus is self-limiting (Fig. I)‘“‘“. Interestingly, although nude mice are immunodeficient, they are able to resist a challenge with influenza virus when previously vaccinated with a recombinant vaccinia virus encoding both IL-2 and the HA gene of influenza virusis. Although the growth of these cytokine-encoding viruses is also highly attenuated in normal mice, i.e. the viruses grow to lower titres and cause none of the pathology associated with v,
